1. Field of the Invention
The present invention relates to an ink-jet printhead. More particularly, the present invention relates to a bubble-jet type ink-jet printhead.
2. Description of the Related Art
Ink-jet printing heads are devices for printing a predetermined color image by ejecting a small droplet of printing ink at a desired position on a recording sheet. Ink ejection mechanisms of an ink-jet printer are generally categorized into two types: an electro-thermal transducer type (bubble-jet type), in which a heat source is employed to form a bubble in ink causing an ink droplet to be ejected, and an electro-mechanical transducer type, in which a piezoelectric crystal bends to change the volume of ink causing an ink droplet to be expelled.
Referring to FIGS. 1A and 1B, a conventional bubble-jet type ink ejection mechanism will now be described. When a current pulse is applied to a heater 12 consisting of resistive heating elements formed in an ink channel 10 where a nozzle 11 is located, heat generated by the heater 12 boils ink 14 to form a bubble 15 within the ink channel 10, which causes an ink droplet 14xe2x80x2 to be ejected.
There are multiple factors and parameters to consider in making an ink-jet printhead having a bubble-jet type ink ejector. First, it should be simple to manufacture, have a low manufacturing cost, and be capable of being mass-produced. Second, in order to produce high quality color images, the formation of minute, undesirable satellite ink droplets that usually trail an ejected main ink droplet must be avoided. Third, when ink is ejected from one nozzle or when ink refills an ink chamber after ink ejection, cross-talk with adjacent nozzles, from which no ink is ejected, must also be avoided. To this end, a back flow of ink in a direction opposite to the direction ink is ejected from a nozzle must be prevented during ink ejection. For this purpose, a second heater 13 as shown in FIGS. 1A and 1B is typically provided to prevent a back flow of the ink 14. The second heater 13 generates heat sooner than the first heater 12, which causes a bubble 16 to shut off the ink channel 10 behind the first heater 12. Then, the first heater 12 generates heat, and the bubble 15 expands to cause the ink droplet 14xe2x80x2 to be ejected. Fourth, for high-speed printing, a cycle beginning kit with ink ejection and ending with ink refill in the ink channel must be carried out in as short a period of time as possible. Fifth, a nozzle and an ink channel for introducing ink to the nozzle must not be clogged by a foreign material or by solidified ink.
The above requirements, however, tend to conflict with one another. Furthermore, the performance of an ink-jet printhead is closely associated with and affected by the structure and design of an ink chamber, an ink channel, and a heater, as well as by the type of formation and expansion of bubbles and the relative size of each component.
FIG. 2 illustrates a perspective, partial cutaway view of a conventional ink-jet printhead showing the internal structure of the ink-jet printhead, and FIG. 3 illustrates a cross-sectional view of the conventional printhead shown in FIG. 2, taken along the line Ixe2x80x94I for explaining how an ink droplet is ejected from the printhead. Referring to FIG. 2, the ink-jet printhead includes a substrate 20, a wall 22 formed on the substrate 20 for providing an ink chamber 26 for containing ink, a heater 23 disposed in the ink chamber 26 for generating heat, and a nozzle plate 21 having an orifice 24 for ejecting an ink droplet. Ink is supplied to the ink chamber 26 through an ink channel 25 and to the orifice 24 in flow communication with the ink chamber 26 by capillary action.
Referring to FIG. 3, in this configuration, if current is applied to the heater 23, the heater 23 generates heat to form a bubble B in ink, thereby filling the ink chamber 26 as shown in FIG. 3. Then, the bubble B expands to exert pressure on the ink within the ink chamber 26 causing an ink droplet 28 to be ejected through the orifice 24.
However, in the ink-jet printhead having the structure described above, a considerable amount of heat generated by the heater 23 is transferred and absorbed into the substrate 20. It is desirable that the heat generated by the heater 23 be used to boil ink and form the bubble B. However, most of the heat is absorbed into the substrate 20, and only a small amount of the heat is actually used to form the bubble B. This means that the heat energy supplied to generate the bubble B is wasted in heating the substrate 20, thereby increasing energy consumption. Also, the ink-jet printhead has a problem in that the temperature of a head is significantly increased as a print cycle runs because the heat transferred to the substrate 20 in turn heats the head system. Furthermore, the heat flow into the substrate 20 causes the ink to be heated or cooled at a lower speed or cycle, thereby increasing the length of the cycle from formation to collapse of the bubble and thus decreasing print speed.
Typically, the amount of ink pushed away from a nozzle by a generated bubble is closely related to the print speed of an ink-jet printhead. In the ink-jet printhead having the conventional structure described above, the amount of ink that is pushed away from the orifice 24 is approximately the same as the amount of ink ejected by the bubble B, thereby making a print cycle longer and thus reducing the print speed of the printhead.
In an effort to solve the above problems, it is a feature of an embodiment of the present invention to provide a bubble-jet type ink-jet printhead configured so that a heater disposed within an ink chamber does not directly contact a substrate and further configured so that an ink channel is disposed inside the substrate thereby consuming less energy in operating the printhead, preventing a backflow of ink, and increasing the printing speed of the printhead.
Accordingly, the present invention provides a bubble-jet type ink-jet printhead including: a substrate; a nozzle plate separated from the substrate by a predetermined distance, the nozzle plate having an orifice for ejecting ink; a wall for closing the space between the substrate and the nozzle plate and for forming an ink chamber filled with ink therebetween; and a heater interposed between the substrate and the nozzle plate for dividing the ink chamber into a main ink chamber disposed above the heater and a secondary ink chamber disposed below the heater, the main ink chamber and the secondary ink chamber generating a main bubble and a secondary bubble, respectively, upon heating of the heater.
Preferably, a groove for forming the secondary ink chamber is formed in the substrate at a location corresponding to the heater. Additionally, it is preferable that the main ink chamber and the secondary ink chamber are in flow communication.
In another embodiment of the present invention, a bubble-jet type ink-jet printhead includes: a substrate; a nozzle plate separated from the substrate by a predetermined distance, the nozzle plate having an orifice for ejecting ink; a wall for closing the space between the substrate and the nozzle plate and for forming an ink chamber filled with ink therebetween; a heater interposed between the substrate and the nozzle plate for dividing the ink chamber into a main ink chamber disposed above the heater and a secondary ink chamber disposed below the heater, the main ink chamber and the secondary ink chamber generating a main bubble and a secondary bubble, respectively, upon heating of the heater and an ink channel connecting the secondary ink chamber to an ink reservoir so that ink is introduced into the secondary ink chamber and then supplied to the main ink chamber.
Preferably, a groove for forming the secondary ink chamber is formed in the substrate at a location corresponding to the heater. It is also preferable that the ink channel is formed at a location corresponding to the central portion of the heater by penetrating the bottom of the secondary ink chamber. Preferably, upper and lower passivation layers are formed above and below the heater, respectively. Also preferably, a portion of the lower passivation layer at a location corresponding to the ink channel is thinner than the upper passivation layer.